1. Field of the Invention
The present invention relates to an optical head provided with separate type fixed and movable optical systems.
2. Description of the Related Arts
One example of a construction of a conventional optical head will be described hereinafter referring to FIG. 18a. FIG. 18a shows one embodiment of a separate type optical head provided with a fixed optical system 1 and a movable optical system 2. In the fixed optical system 1, a light emitted from a semiconductor laser 3 is transmitted or reflected by a first beam splitter 4 and is divided into two lights A and B.
The transmitted light A is reflected on a second beam splitter 5, and is turned in a perpendicular direction by a reflective mirror 6 disposed within the movable optical system 2, and after passing through an objective 7, the Light A irradiates a surface of an optical data recording medium 8 to record data thereon. Then, a reflective light from the recording medium 8 follows the incident optical path reversely to be led again into the fixed optical system 1, and the light is transmitted through the beam splitter 5 to be received by a half-split light receiving element 9. Then, a tracking error signal is detected by using a known push-pull method etc. to carry out a tracking servo.
While, the light B reflected on the beam splitter 4 is led to a half mirror 10. As shown in FIG. 18b, one portion of the light B is transmitted through the half-split light receiving element 11 having two light receiving faces a, b. The rest of the light B is reflected by the half mirror 10 to be led to a light receiving element 12. When the optical axis of the light B is relatively offset with respect to the movable optical system 2, incident light amounts onto the light receiving faces a, b of the light receiving element 11 are different from each other. An output of a differential amplifier 13 is therefore positive or negative. When the optical axis of the light B is relatively inclined with respect to the movable optical system 2, the incident light amounts onto two light receiving faces c, d of the light receiving element 12 are different from each other, and an output of a differential amplifier 14 is positive or negative. Based on the above, an inclination of the optical axis is corrected by servo mechanism (not shown) such that both outputs of the differential amplifier 13, 14 are 0. Thus, the tracking offset can be removed.
In the signal detecting method having the aforementioned conventional construction, two lights i.e. the light A for detecting the signal and the light B for correcting the offset of the optical axis are used. The offset of the optical axis of the light A for detecting the signal is read out by using the light B for correcting the offset of the optical axis instead of the light A. Therefore, an actual offset of the optical axis is not detected and, thereby, not corrected exactly. The slightest inclination in the arrangement position between the two beam splitters 4, 5 prevents the two lights from advancing in parallel to each other toward the movable optical systems. If the two lights A, B are not parallel to each other, exact outputs of the light receiving elements 11, 12 may not be obtained.
Further, the tracking offset caused by the offset of the optical axis is determined by the offset and the inclination of the optical axis on the reflective mirror 6. If the offset of the optical axis is detected by an optical element attached on a casing of the movable optical system 2, correction of the actual offset of the optical axis can not be always performed exactly due to assembling accuracy of the casing and attaching error of the reflective mirror.
In addition to the light receiving element for detecting a tracking error signal, a light receiving element for correcting a tracking error signal is needed. The increase of the number of such light receiving elements leads to also increase the number of electric circuits and adjustment parts and, therefore, the conventional optical head is disadvantage in production cost.